1. Field of the Invention
The invention concerns a system for storing liquid under pressure including a pressurized storage tank and a particular application of this system on spacecraft for storing one or more liquid propellants.
2. Description of the Prior Art
There are many known applications of storage tanks containing liquids at a pressure compatible with their subsequent use. Given the serious damage that can be caused by an exploding pressurized storage tank, these containers are dimensioned to minimize this risk in all planned operating conditions with safety margins in line with applicable regulations.
The storage tank containing the liquid is usually hermetically closed. It generally contains a pressurizing gas. The ratio between the volume of the liquid and the total interior volume of the storage tank is referred to hereinafter as the filling ratio.
The container is closed at a particular pressure and at a particular temperature and with a given filling ratio.
The temperature may vary after the container is closed.
If the temperature increases, the pressure increases. The pressure increase is complicated to calculate as not only does the gas pressure increase with temperature but also the liquid expands, its saturation vapor pressure in the pressurizing gas increases and the container volume changes due to its elasticity.
Generally speaking, however, the pressure increases with the temperature and the rate of increase is directly proportional to the storage tank filling ratio: the pressure in a given container when half full will increase substantially as predicted by the perfect gas laws but the same container if completely full is likely to rupture very quickly for the same rise in temperature, rather like a bottle of water which freezes, as the elasticity of the envelope is not able to withstand the expansion of the incompressible fluid.
The maximum pressure therefore depends on the temperature and the pressure at which the container is closed, on the filling ratio and on the maximum design temperature. The maximum pressure is one of the most important parameters in determining the size of the envelope. The higher the pressure the thicker and therefore the heavier the storage tank must be.
For a given quantity of liquid, a given initial pressure and a given temperature range the storage tank mass is minimal for a particular optimum filling ratio. With a smaller storage tank and therefore a higher filling ratio the storage tank is heavier as it must be thicker. With a larger storage tank the wall thickness may be reduced but the mass is nevertheless increased because of the increased surface area of the envelope.
Spacecraft and satellites routinely employ pressurized storage tanks to transport and use liquids, in particular propellants for the apogee burn and subsequent correction maneuvers. As mass is an extremely critical parameter in spacecraft (up to 100 tonnes of fuel can be required at launch per kilogram of payload put into orbit, and the quantity of propellants can represent more than half the total mass of a geostationary satellite, for example), the mass of the storage tanks is a key factor in the overall mass.
In the case of geostationary satellites, the launch site can be at different latitudes. The French ARIANE launch vehicle is launched from Kourou in Guyana, for example, American launch vehicles are launched from the Kennedy Space Center in Florida, and Chinese launch vehicles are launched from Xichang. Because of the laws of celestial mechanics, different quantities of propellants are required at different launch sites for the same mission and the same mission life, the quantity of propellants increasing with the latitude of the launch site. The ARIANE launch site is optimal because of the low inclination (7.degree. to 10.degree.) of the transfer orbit, as compared with transfer orbit inclinations of 27.degree. to 28.degree. for a launch from Florida.
The quantities of propellants can differ by as much as 20%.
It is routine practise to design satellites to be compatible with different launch vehicles to enable the satellite owners more flexibility in negotiating launch fees and to provide a backup in the event of a serious failure in a family of launch vehicles.
Given the interior complexity of satellite propellant storage tanks (associated with the low gravity and the acceleration that can be required), which usually incorporate relatively delicate capillary retention devices, given also the position of the storage tanks on the satellite and the fact that they are assembled by welding, and given finally the late date at which it may become necessary to change launch vehicles, there is no question of modifying the storage tanks to suit the launch vehicle. Satellites therefore have propellant storage tanks designed and sized to suit the launch vehicle imposing the severest constraints in respect of propellant quantities.
An object of the present invention is to enable optimization of the mass of the propellant storage system according to the differing quantities of propellants, allowing for the launch site, for example, by enabling adaptation of the usable volume of the propellant storage tank at a late stage in the assembly of the satellite.
A more general object of the invention is a system for storing liquid under pressure adapted to withstand temperature variations within a known range and the mass of which can be optimized by means of minor modifications according to the quantity of liquid to be eventually introduced into the system.